Door: kelbytv.com/photoshopkillertips
Get your edges flush and correct with this option.
http://kelbytv.com/photoshopkillertips/2009/10/23/snapping-layers-to-each-other/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+PhotoshopKillerTips+%28Photoshop+Killer+Tips+%28Video%29%29